Occupational safety and health (OSH) is a cross-disciplinary area concerned with protecting the safety, health and welfare of people engaged in work or employment and entrants. As a secondary effect, OSH may also protect co-workers, family members, employers, customers, suppliers, nearby communities, and other members of the public who are impacted by the workplace environment.

Since 1950, the International Labour Organization (ILO) and the World Health Organization (WHO) have shared a common definition of occupational health. It was adopted by the Joint ILO/WHO Committee on Occupational Health at its first session in 1950 and revised at its twelfth session in 1995. The definition reads: "Occupational health should aim at: the promotion and maintenance of the highest degree of physical, mental and social well-being of workers in all occupations; the prevention amongst workers of departures from health caused by their working conditions; the protection of workers in their employment from risks resulting from factors adverse to health; the placing and maintenance of the worker in an occupational environment adapted to his physiological and psychological capabilities; and, to summarize, the adaptation of work to man and of each man to his job."

The reasons for establishing good occupational safety and health standards are frequently identified as:

Moral - An employee should not have to risk injury at work, nor should others associated with the work environment.

Economic - many governments realize that poor occupational safety and health performance results in cost to the State (e.g. through social security payments to the incapacitated, costs for medical treatment, and the loss of the "employability" of the worker). Employing organisations also sustain costs in the event of an incident at work (such as legal fees, fines, compensatory damages, investigation time, lost production, lost goodwill from the workforce, from customers and from the wider community).

Legal - Occupational safety and health requirements may be reinforced in civil law and/or criminal law; it is accepted that without the extra "encouragement" of potential regulatory action or litigation, many organisations would not act upon their implied moral obligations.

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Different states take different approaches to legislation, regulation, and enforcement.

In the European Union, member states have enforcing authorities to ensure that the basic legal requirements relating to occupational safety and health are met. In many EU countries, there is strong cooperation between employer and worker organisations (e.g. Unions) to ensure good OSH performance as it is recognized this has benefits for both the worker (through maintenance of health) and the enterprise (through improved productivity and quality). In 1996 the European Agency for Safety and Health at Work was founded.

Member states of the European Union have all transposed into their national legislation a series of directives that establish minimum standards on occupational safety and health. These directives (of which there are about 20 on a variety of topics, follow a similar structure requiring the employer to assess the workplace risks and put in place preventive measures based on a hierarchy of control. This hierarchy starts with elimination of the hazard and ends with personal protective equipment.

In the UK, health and safety legislation is drawn up and enforced by the Health and Safety Executive and local authorities (the local council) under the Health and Safety at Work etc. Act 1974. Increasingly in the UK the regulatory trend is away from prescriptive rules, and towards risk assessment. Recent major changes to the laws governing asbestos and fire safety management embrace the concept of risk assessment.

OSHA has been regulating occupational safety and health since 1971. Occupational safety and health regulation of a limited number of specifically defined industries was in place for several decades before that, and broad regulations by some individual states was in place for many years prior to the establishment of OSHA.

In Canada, workers are covered by provincial or federal labour codes depending on the sector in which they work. Workers covered by federal legislation (including those in mining, transportation, and federal employment) are covered by the Canada Labour Code; all other workers are covered by the health and safety legislation of the province they work in. The Canadian Centre for Occupational Health and Safety (CCOHS), an agency of the Government of Canada, was created in 1978 by an Act of Parliament. The act was based on the belief that all Canadians had "...a fundamental right to a healthy and safe working environment." . CCOHS is mandated to promote safe and healthy workplaces to help prevent work-related injuries and illnesses.

A risk is a combination of the probability that a particular outcome will occur and the severity of the harm involved.

“Hazard”, “risk”, and “outcome” are used in other fields to describe e.g. environmental damage, or damage to equipment. However, in the context of OSH, “harm” generally describes the direct or indirect degradation, temporary or permanent, of the physical, mental, or social well-being of workers. For example, repetitively carrying out manual handling of heavy objects is a hazard. The outcome would be a musculoskeletal disorder (MSD). The risk can be expressed numerically, (e.g. a 0.5 or 50/50 chance of the outcome occurring during a year), qualitatively as "high/medium/low", or using a more complicated classification scheme.

Modern occupational safety and health legislation usually demands that a risk assessment be carried out prior to making an intervention. This assessment should:

Identify the hazards

Identify all affected by the hazard and how

Evaluate the risk

Identify and prioritise the required actions

The calculation of risk is based on the likelihood or probability of the harm being realised and the severity of the consequences. This can be expressed mathematically as a quantitative assessment (by assigning low, medium and high likelihood and severity with integers and multiplying them to give a risk factor), or as a description of the circumstances by which the harm could arise i.e. qualitative.

The assessment should be recorded and reviewed periodically and whenever there is a significant change to work practices. The assessment should include practical recommendations to control the risk. Once recommended controls are implemented, the risk should be re-calculated to determine of it has been lowered to an acceptable level. Generally speaking, newly introduced controls should lower risk by one level, i.e, from high to medium or from medium to low

The precautionary principle is an increasingly used method for reducing potential chemical or biological OSH risks.